In general, a hard disk drive (HDD) includes a suspension substrate having a magnetic head slider mounted thereon that writes and reads data to/from a disc that stores data. The suspension substrate has a plurality of wiring line layers, and a plurality of wiring line pads provided near a mounting region where the magnetic head slider is mounted. The wiring line pads are respectively connected to the wiring line layers. By the wiring line pads being connected to slider pads of the magnetic head slider, passing of data is performed on the magnetic head slider.
In recent years, with an increase in the capacity and information transmission rate of HDDs, there has been a demand for wiring lines of a suspension substrate to be larger in number, finer, and stacked. For example, to suppress the occurrence of crosstalk, there has been proposed a suspension substrate including a metal substrate, a first insulating layer formed on the metal substrate, a pair of first wiring line layers formed on the first insulating layer with a predetermined spacing therebetween, a second insulating layer formed to cover the first wiring line layers, and a pair of second wiring line layers formed on the second insulating layer with a predetermined spacing therebetween (see, for example, Patent Literatures 1 and 2).
However, in such a conventional suspension substrate including stacked wiring lines, the second insulating layer is formed on the first insulating layer and the first wiring line layers and thus has a shape that follows differences in level caused by the surface of the first insulating layer and the surfaces of the first wiring line layers, thereby causing irregularities on the surface of the second insulating layer. When second wiring line layers are formed on such an irregular surface of the second insulating layer, displacement of the positions of the second wiring line layers may occur or a defect in the pattern of the second wiring line layers may occur. As a result, there is a problem that the impedance of the first wiring line layers and the second wiring line layers becomes unstable.